Mitochondrial ascorbic acid is responsible for enhanced susceptibility of U937 cells to the toxic effects of peroxynitrite

The effect of oxidative and reductive stress on semen parameters and functions of physiologically normal human spermatozoa

Both oxidative stress (OS) and reductive stress (RS) are the two extreme facets of redox imbalance that can have deleterious effects on sperm function. However, there is a lack of information on the physiological range of oxidation-reduction potential (ORP). The aim of this study was to investigate the effect of OS and RS on functions and associated molecular changes in normal spermatozoa in order to establish the physiological range of ORP. In the current study, total and progressive motility remained unchanged in spermatozoa exposed to ORP values 0.33 and 0.72 mV/10⁶ sperm/mL. However, a significant (P < 0.05) decline in total and progressive motility were observed at ORP values 1.48, 2.75, -11.24, -9.76 and -9.35 mV/10⁶ sperm/mL. Sperm vitality also decreased significantly (P < 0.0001) at 2.75, -11.24 and -9.76 mV/10⁶ sperm/mL. Spermatozoa exposed to ORP levels 2.75 and -11.24 mV/10⁶ sperm/mL showed a significant (P < 0.01) decrease in mitochondrial membrane potential. Intracellular reactive oxygen species (iROS) production increased (P < 0.05) in spermatozoa exposed to ORP levels of 1.48 and 2.75 mV/10⁶ sperm/mL, while iROS decreased (P < 0.05) at ORP levels -9.76 and -11.24 mV/10⁶ sperm/mL. No significant change in sperm DNA fragmentation was noted in sperm exposed to OS/RS and the values were below the reference range (<19.25%). Western blot analysis revealed decreased expression of CV-ATPA, CIII-UQCRC2 and CIV-MTCO1 proteins at 60 and 120 min (P < 0.05) in both OS and RS conditions. This is the first study to report physiological range of ORP (between -9.76 and 1.48 mV/10⁶ sperm/mL) and to elucidate the role of altered expression of oxidative phosphorylation (OXPHOS) complexes proteins in mediating detrimental effects of oxidative and reductive conditions on sperm functions. A decreased expression of OXPHOS proteins and associated mitochondrial dysfunction contributes to decreased sperm motility and vitality under oxidative and reductive stress.

Concentration Does Matter: The Beneficial and Potentially Harmful Effects of Ascorbate in Humans and Plants

SIGNIFICANCE

Ascorbate (Asc) is an essential compound both in animals and plants, mostly due to its reducing properties, thereby playing a role in scavenging reactive oxygen species (ROS) and acting as a cofactor in various enzymatic reactions. Recent Advances: Growing number of evidence shows that excessive Asc accumulation may have negative effects on cellular functions both in humans and plants; inter alia it may negatively affect signaling mechanisms, cellular redox status, and contribute to the production of ROS via the Fenton reaction.

CRITICAL ISSUES

Both plants and humans tightly control cellular Asc levels, possibly via biosynthesis, transport, and degradation, to maintain them in an optimum concentration range, which, among other factors, is essential to minimize the potentially harmful effects of Asc. On the contrary, the Fenton reaction induced by a high-dose Asc treatment in humans enables a potential cancer-selective cell death pathway.

FUTURE DIRECTIONS

The elucidation of Asc induced cancer selective cell death mechanisms may give us a tool to apply Asc in cancer therapy. On the contrary, the regulatory mechanisms controlling cellular Asc levels are also to be considered, for example, when aiming at generating crops with elevated Asc levels.

Low Concentrations of Arsenite Target the Intraluminal Inositol 1, 4, 5-Trisphosphate Receptor/Ryanodine Receptor Crosstalk to Significantly Elevate Intracellular Ca2

Arsenite is an established human carcinogen that induces cytotoxic and genotoxic effects through poorly defined mechanisms involving the formation of reactive oxygen species (ROS) and deregulated Ca²⁺ homeostasis. We used variants of the U937 cell line to address the central issue of the mechanism whereby arsenite affects Ca²⁺ homeostasis. We found that 6-hour exposure to the metalloid (2.5 μM), although not associated with an immediate or delayed toxicity, causes a significant increase in the intracellular Ca²⁺ concentration ([Ca²⁺]_i) through a mechanism characterized by the following components: 1) it was not affected by ROS produced under the same conditions; 2) a small amount of Ca²⁺ was mobilized from the inositol-1,4,5-trisphosphate receptor (IP₃R), and this response was not augmented by greater concentrations of the metalloid; 3) large amounts of Ca²⁺ were instead dose dependently mobilized from the ryanodine receptor (RyR) in response to IP₃R stimulation; 4) the cells maintained an intact responsiveness to agonist-stimulated Ca²⁺ mobilization from both channels; 5) arsenite, even at 5-10 µM, failed to directly mobilize Ca²⁺ from the RyR; and 6) arsenite failed to enhance Ca²⁺ release from the RyR under conditions in which the [Ca²⁺]_i was increased by either RyR agonists or ionophore-stimulated Ca²⁺ uptake. We therefore conclude that arsenite elevates the [Ca²⁺]_i by directly targeting the IP₃R and its intraluminal crosstalk with the RyR. This mechanism likely mediates mitochondrial superoxide formation, downstream damage on various biomolecules (including genomic DNA), and mitochondrial dysfunction/apoptosis eventually occurring after longer incubation to, or exposure to greater concentrations of, arsenite.

Differentiation of Promonocytic U937 Cells to Monocytes Is Associated with Reduced Mitochondrial Transport of Ascorbic Acid

Growth of promonocytic U937 cells in the presence of DMSO promotes their differentiation to monocytes. After 4 days of culture in differentiating medium, these cells ceased to proliferate, displayed downregulated ryanodine receptor expression, and responded to specific stimuli with enhanced NADPH-oxidase-derived superoxide formation or cytosolic phospholipase A₂-dependent arachidonic acid release. We found that the 4-day differentiation process is also associated with downregulated SVCT2 mRNA expression, in the absence of apparent changes in SVCT2 protein expression and transport rate of ascorbic acid (AA). Interestingly, under the same conditions, these cells accumulated lower amounts of the vitamin in their mitochondria, with an ensuing reduced response to external stimuli sensitive to the mitochondrial fraction of AA. Further analyses demonstrated an unexpected increase in mitochondrial SVCT2 protein expression, however, associated with reduced SVCT2-dependent AA uptake in isolated mitochondria. A decrease in the transporter Vmax, with no change in affinity, was found to account for this response. Differentiation of promonocytic cells to monocytes is therefore characterized by decreased SVCT2 mRNA expression that, even prior to the onset of SVCT2 protein downregulation or apparent changes in plasma membrane transport activity, impacts on the mitochondrial accumulation of the vitamin through a decreased Vmax of the transporter.

Intramitochondrial Ascorbic Acid Enhances the Formation of Mitochondrial Superoxide Induced by Peroxynitrite via a Ca2+-Independent Mechanism

Exposure of U937 cells to peroxynitrite promotes mitochondrial superoxide formation via a mechanism dependent on both inhibition of complex III and increased mitochondrial Ca²⁺ accumulation. Otherwise inactive concentrations of the oxidant produced the same maximal effects in the presence of either complex III inhibitors or agents mobilizing Ca²⁺ from the ryanodine receptor and enforcing its mitochondrial accumulation. l-Ascorbic acid (AA) produced similar enhancing effects in terms of superoxide formation, DNA strand scission and cytotoxicity. However, AA failed to enhance the intra-mitochondrial concentration of Ca²⁺ and the effects observed in cells supplemented with peroxinitrite, while insensitive to manipulations preventing the mobilization of Ca²⁺, or the mitochondrial accumulation of the cation, were also detected in human monocytes and macrophages, which do not express the ryanodine receptor. In all these cell types, mitochondrial permeability transition-dependent toxicity was detected in cells exposed to AA/peroxynitrite and, based on the above criteria, these responses also appeared Ca²⁺-independent. The enhancing effects of AA are therefore similar to those mediated by bona fide complex III inhibitors, although the vitamin failed to directly inhibit complex III, and in fact enhanced its sensitivity to the inhibitory effects of peroxynitrite.

Cytotoxic Potential of Bacillus cereus Strains ATCC 11778 and 14579 Against Human Lung Epithelial Cells Under Microaerobic Growth Conditions

Bacillus cereus, a food poisoning bacterium closely related to Bacillus anthracis, secretes a multitude of virulence factors including enterotoxins, hemolysins, and phospholipases. However, the majority of the in vitro experiments evaluating the cytotoxic potential of B. cereus were carried out in the conditions of aeration, and the impact of the oxygen limitation in conditions encountered by the microbe in natural environment such as gastrointestinal tract remains poorly understood. This research reports comparative analysis of ATCC strains 11778 (BC1) and 14579 (BC2) in aerobic and microaerobic (static) cultures with regard to their toxicity for human lung epithelial cells. We showed that BC1 increased its toxicity upon oxygen limitation while BC2 was highly cytotoxic in both growth conditions. The combined effect of the pore-forming, cholesterol-dependent hemolysin, cereolysin O (CLO), and metabolic product(s) such as succinate produced in microaerobic conditions provided substantial contribution to the toxicity of BC1 but not BC2 which relied mainly on other toxins. This mechanism is shared between CB1 and B. anthracis. It involves the permeabilization of the cell membrane which facilitates transport of toxic bacterial metabolites into the cell. The toxicity of BC1 was potentiated in the presence of bovine serum albumin which appeared to serve as reservoir for bacteria-derived nitric oxide participating in the downstream production of reactive oxidizing species with the properties of peroxynitrite. In agreement with this the BC1 cultures demonstrated the increased oxidation of the indicator dye Amplex Red catalyzed by peroxidase as well as the increased toxicity in the presence of externally added ascorbic acid.

In silico aided thoughts on mitochondrial vitamin C transport

The huge demand of mitochondria as the quantitatively most important sources of ROS in the majority of heterotrophic cells for vitamin C is indisputable. The reduced form of the vitamin, l-ascorbic acid, is imported by an active mechanism requiring two sodium-dependent vitamin C transporters (SVCT1 and SVCT2). The oxidized form, dehydroascorbate is taken up by different members of the GLUT family. Because of the controversial experimental results the picture on mitochondrial vitamin C transport became quite obscure by the spring of 2014. Thus in silico prediction tools were applied in aid of the support of in vitro and in vivo results. The role of GLUT1 as a mitochondrial dehydroascorbate transporter could be reinforced by in silico predictions however the mitochondrial presence of GLUT10 is not likely since this transport protein got far the lowest mitochondrial localization scores. Furthermore the possible roles of GLUT9 and 11 in mitochondrial vitamin C transport can be proposed leastwise on the base of their computational localization analysis. In good concordance with the newest experimental observations on SVCT2 the mitochondrial presence of this transporter could also be supported by the computational prediction tools.

A novel biological role of dehydroascorbic acid: Inhibition of Na(+)-dependent transport of ascorbic acid

A U937 cell clone, in which low micromolar concentrations of ascorbic acid (AA) and dehydroascorbic acid (DHA) are taken up at identical rates, was used to investigate possible interactions between transport systems mediating cellular uptake of the two forms of the vitamin. Results obtained with different experimental approaches showed that DHA potently and reversibly inhibits AA uptake through Na(+)-AA cotransporters. Hence, a progressive increase in extracellular DHA concentrations in the presence of a fixed amount of AA caused an initial decrease in the net amount of vitamin C accumulated, and eventually, at higher levels, it caused an accumulation of the vitamin solely based on DHA uptake through hexose transporters. DHA-dependent inhibition of AA uptake was also detected in various other cell types. Taken together, our results provide evidence of a novel biological effect mediated by concentrations of DHA compatible with those produced at inflammatory sites.